Presently known art in the manufacture of textured high pressure decorative laminates is as follows:
An assembly consisting of the following is placed in a flat-bed hydraulic press, whereupon heat and pressure are applied:
-- Iron PA1 -- Paper cushion PA1 -- Stainless steel plate PA1 -- Laminated aluminum-paper caul stock PA1 -- Overlay sheet PA1 -- Decor sheet PA1 -- Layers of phenolic-kraft core stock PA1 -- Separator sheet (center line)
The assembly is symmetrical about the center line, therefore, only the upper half is shown. For a complete description of high pressure laminating, see Encylopedia of Polymer Science and Technology, John Wiley and Sons, Vol. 8, pps. 121-163 (1968).
The aluminum-paper caul stock now used, consists of alloy No. 1100 usually in fully annealed state (-0 temper), but also frequently in a harder state such as H-18 or H-19 (full hard). The aluminum foil portion may have a matte finish, a polished finish or a mill finish on its exposed face. The aluminum is bonded to a paper of about 40 lbs./ream by means of casein adhesives. This operation is performed by the caul stock supplier. (See I.O. Robertson, Jr. "Use of Aluminum Foil Release Sheet in Decorative Laminates", TAPPI, Vol. 55, pps. 1341-4, September, 1972.)
When fully annealed alloy No. 1100 in a thickness of 0.0005 inches is laminated matte side out to a 40 lb./ream machine glazed natural kraft paper, it is often called "No. 6 Caul Stock". When used in the above assembly, it will produce a high pressure laminate with an arithmetic average (AA) roughness of about 90 .+-. 20 microinches. See "Surface Texture" (ASA B46.1-1962) American Society of Mechanical Engineers, New York, 17, NY. Its gloss will be 5.degree.-10.degree. measured on a Gardner 60.degree. glossmeter and will depend upon the particular specimen of caul stock chosen. Most rolls will produce laminates of about 6.degree.-8.degree. gloss, but others will produce values up to 10.degree. or higher.
Because of this variation in gloss, it is customary to finish these laminates by the process of "dull rubbing". In this process the laminate is passed through a machine in which its decorative surface is contacted by a number of rotating, cylindrical, nylon fiber brushes. The brushes are flooded with a slurry of pumice and water which abrades the laminate surface and reduces its gloss by about 1.degree.-5.degree. depending upon the force which is used to bring the brush into contact with its surface. Typically, a laminate emerging from the press with a gloss of 6.5 would be dull rubbed with light contact pressure and abraded to a gloss of about 5.5. A laminate having a gloss of 10, however, would require a substantially higher force to bring the brush into very firm contact with the laminate. Even then the reduction in gloss might be only about 3.degree., say to about 7.degree..
If the aluminum foil caul stock were more uniform, it would produce laminates having glosses within the range of 6 .+-. 2, which is commercially desirable. The dull rubbing could then be eliminated which would reduce the cost of manufacture. Additionally, the micro-scratches imparted to the laminate surface during dull rubbing render it less resistant to staining (see Standards Publication LD1-1971, Laminated Thermosetting Decorative Sheets, National Electrical Manufacturers Association, New York, N.Y. 10017 (1971) for method of test). This is due to the fact that thousands of microcracks are introduced into the surface into which very small particles of staining or soiling materials may enter and lodge tenaciously.
Laminates made with the use of No. 6 Caul Stock have a distinctive texture which is often referred to as a "suede" finish, more by custom than any resemblance to a true suede leather.
The suede texture has proven to be of special importance because it is attractive to the touch, yet serves to overcome "telegraphing" of joints, glue lines, coarse grain and other discontinuities which may occur in modern furniture structures, especially those which use frame construction as opposed to solid panels, e.g. in a table top. "Telegraphing" is used in the industry to designate the ability of a plastic sheet to reproduce in its upper surface whatever texture may be possessed by the substrate upon which it rests. Thus, for many years, the best practice in mounting high pressure laminates was to use hardwood faced plywood, usually birch or maple. With the advent of reconstructed wood particleboard for laminate substrates, it became the practice to use three layer construction which featured a layer of "fines" on the bondable surfaces to eliminate "telegraphing".
Furniture manufacturers learned that the suede surface is only mildly textured and that the ultimate furniture user will not be unduly aware of the depth of the texture. For instance, such a textured laminate will provide a suitable writing surface if used as a desk top.
Because of this unique dimension in texture, i.e. one which is coarse enough to hide telegraphing, yet smooth enough to be accepted as planar, this "mini texture" has enjoyed great popularity and now accounts for over 50% of all the commercial laminates produced in the U.S.
The amount of aluminum foil used in laminate production is very substantial so that the aggregate value of such foil consumed annually is very great. Laminate manufacturers have, therefore, sought materials which would be lower in cost on a unit basis, since the annual savings to be realized are of an important magnitude. One manner in which this may be accomplished is to replace the aluminum caul stock with a sheet of paper which is coated with a material which will fill surface voids and impart a texture of sufficient smoothness and gloss to the surface of a decorative laminate. Textured laminates may have useful gloss levels ranging from about six units to about 25-30 units.
It is necessary for the texturing sheet to release readily after it has been used on the laminate, hence a release agent of some type is often applied in a very thin layer over the basic texturing coat. In U.S. Pat. No. 3,946,135 to Peterson, such a release paper is described. Said release paper comprises a first coat constituting a clay-adhesive base coat and a second coat of a thermosetting resin.
The need for two coats makes the cost of the coated paper greater since one must either (i) mount the paper on the coater twice (ii) have access to a two station coater (which is more costly) or (iii) buy the paper with one coat already applied (which again is more costly).
It can be readily appreciated that a single coating which accomplishes both the filling and texturing of the paper as one function and permits ready release from a decorative melamine laminate as a second function is simpler and cheaper than the two coat variety.
Single coat release sheets can be prepared by the use of an additive in the texturing coat which makes it releasable upon drying. Such a material is called an "internal release agent" because it is contained internally in the texturing coat as it is applied. Release agents which are applied as a distinctly second coat are called "external release agents" since they never enter the central volume of the texturing coat. (See "Selecting Mold Release Agents for FRP" J. W. Waldeck, Plastics World, pps. 40-42, Jan. 19, 1976.)
As noted by Waldeck, the precise mechanism by which internal release agents operate is not known. Without being restricted to any particular mechanism herein, it appears that relationships between the solubility of the internal release agent in the carrier vehicle (in this case the texturing coat) as applied and as it dries, along with the ability of the release agent to repel the components of the object to be released (in this case a melamine resin surfaced decorative laminate) are the determining factors in the effectiveness of the coating. It is known that the release agent is initially soluble in the texturing coat resin and its solvent system. As the texturing coat loses solvent during drying, and/or as the texturing resin changes character during thermosetting, it is believed the release agent becomes less soluble and thereupon migrates to and concentrates at the surface of the texturing coat. Later when the texture-release paper is laid adjacent to the laminate surface, the concentration of release agent at the coated surface prevents adherence of the two contacting surfaces.